The well-known DIMM (Dual In-line Memory Module) board has been used for years, in various forms, to provide memory expansion. A typical DIMM includes a conventional PCB (printed circuit board) with memory devices and supporting digital logic devices mounted on both sides. The DIMM is typically mounted in the host computer system by inserting a contact-bearing interface edge of the DIMM into an edge connector socket. Systems that employ DIMMs provide limited space for such devices and conventional DIMM-based solutions have typically provided only a moderate amount of memory expansion.
As die sizes increase, the limited surface area available on conventional DIMMs limits the number of devices that may be carried on a memory expansion module devised according to conventional DIMM techniques. Further, as bus speeds have increased, fewer devices per channel can be reliably addressed with a DIMM-based solution. For example, 288 ICs or devices per channel may be addressed using the SDRAM-100 bus protocol with an unbuffered DIMM. Using the DDR-200 bus protocol, approximately 144 devices may be addressed per channel. With the DDR2-400 bus protocol, only 72 devices per channel may be addressed. This constraint has led to the development of the fully-buffered DIMM (FB-DIMM) with buffered C/A and data in which 288 devices per channel may be addressed. With the FB-DIMM, not only has capacity increased, pin count has declined to approximately 69 signal pins from the approximately 240 pins previously required.
This improvement has, however, come with some cost. The basic principle of systems that employ FB-DIMM relies upon a point-to-point or serial addressing scheme rather than the parallel multi-drop interface that dictates non-buffered DIMM addressing. That is, one DIMM is in point-to-point relationship with the memory controller and each DIMM is in point-to-point relationship with adjacent DIMMs. Consequently, as bus speeds increase, the number of DIMMs on a bus will decline as the discontinuities caused by the chain of point-to-point connections from the controller to the “last” DIMM become magnified in effect as speeds increase.
A variety of techniques and systems for enhancing the capacity of DIMMs and similar modules are known. For example, multiple die may be packaged in a single IC package. A DIMM module may then be populated with such multi-die devices. However, multi-die fabrication and testing is complicated and few memory and other circuit designs are available in multi-die packages.
Other techniques have populated FR4 circuit boards with stacks comprised of packaged integrated circuits. Others techniques have employed daughter cards to increase module capacities. Typically, however, as more circuitry is aggregated on a circuit module, thermal issues become more prominent.
Thermally efficient solutions have been offered by Staktek Group L.P., the assignee of the present invention. For example, Staktek has devised a number of circuit module designs that dispose IC-populated flexible circuitry about a thermally-conductive core to provide a thin and thermally efficient circuit module that may supplant traditional DIMMs such as, for example, registered DIMMs as well as FB-DIMMs to name a few modules that may employ such technologies.
Alternatives to traditional FR4 cored circuit modules have found profitable employment in applications where thermal performance is a significant consideration. Consequently, new designs that provide the advantages of thermal performance with readily understood and inexpensive materials are welcome in the field.